It is known practice to use positively driven contacts for safety-relevant electromechanical switching operations, for example with relays.
Such relays usually comprise a plurality of contacts. At least one contact is mechanically connected to all other contacts in order to be able to monitor the other contacts. The monitoring contact is usually “normally closed” and the make contacts are usually “normally open”. “Normally open” or “normally closed” means that this switching position is assumed in the de-energized state, that is to say there is an open or closed position. The monitoring contact therefore always has the opposite switching position to the make contacts.
A monitoring contact makes it possible to determine whether the relay for the operating circuit is operating correctly. If the make contacts are open, the monitoring contact must be closed. Certified relays, in which it is guaranteed that the make contacts are open when the monitoring contact is closed and vice versa, are regularly used. This makes it possible to achieve a required safety level, for example 4.
The prior art also discloses solutions for monitoring relay contacts without a positively driven monitoring contact. DE 27 29 480 A1 discloses a method for monitoring and/or extending switchgear assemblies with contacts which are connected in series in operating circuits or electric circuits, for example for processing systems. In this prior art approach, the respective switching state of the contact is detected without any reaction using a monitoring circuit which is connected in parallel. A contactor is actuated depending on the switching state. A relay is provided in order to interrupt the circuit so that signals from the monitoring circuit do not adversely affect the contactor. However, the manner in which it can be ensured that the relay which decouples the power supply operates correctly remains open in this arrangement.
Another proposed solution is described in the European patent specification EP 0 681 310 B1. A monitoring circuit is provided in parallel with two relays which are connected in series. A filter circuit decouples the monitoring circuit.